Project Severe Weather Archive of the Philippines (SWAP) Part 2: Baseline Climatology of Close Proximity Soundings in Hailstorm Environments across Luzon, Philippines

TL;DR

This paper builds a baseline climatology of hail-bearing environments over Luzon by leveraging 171 proximity soundings (2005–2024) from Project SWAP DR3 and ERA5 reanalysis to quantify thermodynamic, kinematic, and moisture conditions. It introduces HE1–HE3 environmental regimes and shows that high CAPE with weak low-level shear, coupled with strong cloud-depth shear (via BWD$_{LCL-EL}$) and substantial moisture, best discriminates hail-prone environments in the tropics. The study finds two recurring tropical setups—easterly and westerly—with distinct hodograph shapes but similar overall instability, and demonstrates that combining W$_{MAX}$ with cloud-depth shear outperforms DLS alone in identifying hail potential. The results offer practical discriminators for operational forecasting in Luzon and potentially countrywide, and lay groundwork for extending tropical hail climatology to other regions under Project SWAP.

Abstract

The environments of severe thunderstorms that produced hail were examined using 171 proximity soundings (2005-2024) archived in the 3rd Data Release of Project SWAP. These soundings were categorized based on their geographical occurrence into three hail-prone environments across Luzon, Philippines. For each case, key parameters describing instability, vertical wind shear, and moisture were calculated to assess the conditions for hail production. The probability of hail occurrence, expressed as a function of W$_{\text{MAX}}$ ($\sqrt{2 \times \text{CAPE}}$) and 0-6 km bulk shear (DLS), revealed patterns distinct from those reported in other regions. Hail events in Luzon were most likely under high CAPE conditions, where boundary-layer moisture was sufficient, mid- and low-level lapse rates were steep, and lifting condensation levels were high. Surprisingly, weak DLS was common across Luzon hail environments, diverging from existing severe weather climatologies, yet large DCAPE indicated environments conducive to damaging wind events. When DLS was replaced with the shear magnitude between the cloud base and equilibrium level, the probability of hail occurrence increased, better aligning with global severe weather climatologies. This finding is supported by hodograph analyses, which show largely unidirectional wind profiles: strong speed shear aloft but weak directional shear in the low-levels. Parameters such as W$_{\text{MAX}}$SHEAR, W$_{\text{MAX}}$SHEAR$_{\text{LCL-EL}}$, and BWD$_{\text{LCL-EL}}$ emerge as potential discriminators between non-severe and severe thunderstorms capable of producing hail, and as useful metrics for assessing convective storm severity in Luzon and possibly countrywide. Finally, two recurring severe setups conducive to hail were identified: (1) an easterly regime associated with trade winds, and (2) a westerly regime linked to the Asian summer monsoon.

Figures (19)

• Figure 1: (a) Spatial Distribution of Hail Events across Luzon, Philippines. Red circles are events for HE1, Green circles are associated with HE2, and Blue circles are for HE3. (b) 12 yr Climatology of Thunder Hours (hr yr$^{-1}$) at P95 (contour) and average (dashed lines).
• Figure 2: 2D Histogram (joint distribution) of Hail Events across Luzon, Philippines.
• Figure 3: Similar to Figure 2, but for Thunder counts, as proxy for thunderstorm events, across Luzon, Philippines.
• Figure 4: Monthly Distribution of Hail Events (counts mon$^{-1}$) and Thunder Hours (hr mon$^{-1}$) across Luzon, Philippines in different Environments associated in this study. Yellow lines correspond to the average (solid line) and P95 (dashed line) Thunder Hours per month. (a) Total Hail Events across Luzon, (b) HE1, (c) HE2, (d) HE3.
• Figure 5: Raincloud diagrams for (a) MUCAPE, (b) 0-3 km MUCAPE, and (c) MUCAPE$_{\text{HGL}}$ associated with HE1 (Red), HE2 (Green), and HE3 (Blue). The box and whiskers cover the 25th–75th percentiles and the median values are marked by the horizontal line within each box.